Description (from Applicant's abstract): Secondary lymphoid tissues such as lymph nodes (LN) are known to be major sites of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) replication at early and late stages of infection. Despite the presumed importance of the LN compartment as a site of virus production, it is surprising how little is known about the biology of viruses harbored in this microenvironment. The overall goal of this proposal is to characterize a highly pathogenic molecular variant of SIVMne that was directly cloned from uncultured LN cells. Interestingly, this variant has the unique capacity of replicate in nonstimulated peripheral blood cells. Specifically, we will test the hypothesis that SIV may become more pathogenic by acquiring a greater capacity to modulate and respond to T-cell activation signals for efficient replication. We also hypothesize that mutations in the gag-pol region of the virus can influence pathogenicity. In this proposal, we will compare the biological properties of the LN-derived virus with other well-characterized molecular clones of SIVMne in different primary macaque cells to determine how the LN-derived variant interacts with host cells to replicate efficiently. Furthermore, we will construct interviral recombinant viruses aggressively and kill CD4+ T-cells. The following aims have been proposed to address our hypotheses: AIM 1) To characterize the cellular interactions and expression of cytokines that support replication of the LN-derived SIVMne variant in nonstimulated macaque peripheral blood mononuclear cells; To determine the tropism of the LN-derived SIVMne variant for macaque CD4+ T-cell subsets, monocyte/macrophages, and dendritic cells. To examine the cytopathic effects of the LN-derived variant in different subsets of T-cells. AIM 2) To characterize the cellular activation requirements for the LN-derived SIVMne variant to undergo post-entry events in host T-cells. AIM 3) To identify the determinant(s) within the LN-derived SIVMne variant that confers the ability to replicate in nonstimulated macaque peripheral blood mononuclear cells. To identify the determinant(s) that increases cytopathicity of the virus for CD4+ T-lymphocytes. These studies may provide greater insights into the regulation of viral replication and into the selection of viral variants that impact the progression of AIDS.